I. Field of the Invention
This invention relates to a novel blend of bituminous and synthetic rubber components which provides a superior low temperature sealing adhesive. More particularly, the invention relates to a self-sealing composition for roofing shingles and shingles having the composition thereon.
II. Description of the Prior Art
Many blends of bituminous material have been proposed which meet certain specifications but which are lacking in others, particularly specifications for roofing adhesives. Among the requirements for a self-sealing adhesive are resistance to crush when the shingles are packed in stacks for shipment, a relatively low melting temperature to permit self-sealing without the application of heating equipment and a strong bond between the joined surfaces which has high wind resistance and good low temperature stability. Other important considerations include good resistance to photo-oxidation; in particular, the ability to retain adhesive properties after exposure of the adhesive to sunlight for more than two hours. While certain of these characteristics are found in conventional blends, a combination of all of the above-mentioned properties has not been attained.
For example, U.S. Pat. No. 3,978,014 describes blends having high aromaticity and at least 8% rubber content. While these blends exhibit good thermal stability, their self-sealing properties are relatively poor. Also, because or their high aromaticity, these blends are readily degraded by exposure to UV light during roof installation. U.S. Pat. No. 4,559,267 also discloses roofing adhesive blends. However these include substantial amounts of bitumen roofers flux in their composition which produces a tacky sealant leading to rider granule problems and packaging difficulties as exemplified by low squash resistance and premature adhesion of shingles when shipped in bundles and during installation.
Another problem with the prior art adhesive compositions is that they are essentially "contact adhesives". In other words, the primary strength of the bond formed by the adhesive composition is at the surface of the shingle. Thus, while the adhesives are relatively soft, they exhibit extremely slow flow properties at temperatures of 90.degree. to 100.degree. F. This means that the adhesive does not flow into the shingle and the bond remains essentially at the surface thereof. A stronger bond is formed between shingle layers when the adhesive composition flows partially into the contacting face of the shingle. Such an adhesive might be referred to as a "migrating" adhesive. Consequently, it is desired to have such a "migrating" property at low temperatures of 90.degree. to 100.degree. F. in order to provide stronger bonds and better wind resistance.
U.S. Pat. No. 4,755,545 describes a self-sealing adhesive blend composed of a bituminous component of 100 percent of a road paving grade asphalt or a blend of such a road paving asphalt with a hard resin asphalt, 1 to 6% by weight of a thermoplastic styrene/butadiene block copolymer and 4 to 40% by weight of an inert filler. This composition does provide some improvement with respect to squash resistance, reduced tackiness and improved low temperature stability. It is pointed out that the bituminous component must have an ACI of less than 0.26 and it is essential that the asphaltene content be less than 20%. The road paving asphalt is a material having a penetration at 77.degree. F. of between 40-80 dmm and a softening point of between 110.degree. F. and 130.degree. F.
The composition of U. S. Pat. No. 4,755,545 presents additional problems with respect to processing such as, mixing difficulty in the dissolution of the elastomer thus requiring more elaborate equipment, e.g., homogenizers, high shear mixers, and the like.